In drilling a borehole in the earth, such as for the recovery of hydrocarbons or for other applications, it is conventional practice to connect a drill bit on the lower end of an assembly of drill pipe sections which are connected end-to-end so as to form a “drill string.” FIG. 1 includes a drilling installation having a drilling rig 10 at the surface 12 of a well, supporting a drill string 14. The drill string includes a bottom hole assembly 26 (commonly referred to as a “BHA”) coupled to the lower end of the drill string 14. The BHA includes the drill bit 32, which rotates to drill the borehole. As the drill bit 32 operates, drilling fluid or mud is pumped from a mud pit 34 at the surface into the drill pipe 24 and to the drill bit 32. After flowing through the drill bit 32, the drilling mud rises back to the surface, where it is collected and returned to the mud pit 34 for filtering. The mud, in the process of filtering into the formation, leaves a mudcake on the wall of the hole.
Modern oil field operations demand a great quantity of information relating to the parameters and conditions encountered downhole. Such information typically includes characteristics of the earth formations traversed by the wellbore, in addition to data relating to the size and configuration of the borehole itself. The collection of information relating to conditions downhole, which commonly is referred to as “logging,” can be performed by several methods.
Logging has been known in the industry for many years as a technique for providing information regarding the particular earth formation being drilled. In conventional oil well wireline logging, a probe or “sonde” is lowered into the borehole after some or all of the well has been drilled, and is used to determine certain characteristics of the formations traversed by the borehole. The sonde may include one or more sensors to measure parameters downhole and typically is constructed as a hermetically sealed steel cylinder for housing the sensors, which hangs at the end of a long cable or “wireline.” The cable or wireline provides mechanical support to the sonde and also provides an electrical connection between the sensors and associated instrumentation within the sonde, and electrical equipment located at the surface of the well. Normally, the cable supplies operating power to the sonde and is used as an electrical conductor to transmit information signals from the sonde to the surface, and control signals from the surface to the sonde. In accordance with conventional techniques, various parameters of the earth's formations are measured and correlated with the position of the sonde in the borehole, as the sonde is pulled uphole.
Designs for measuring conditions downhole and the movement and the location of the drilling assembly, contemporaneously with the drilling of the well, have come to be known as “measurement-while-drilling” techniques, or “MWD.” Similar techniques, concentrating more on the measurement of formation parameters of the type associated with wireline tools, commonly have been referred to as “logging while drilling” techniques, or “LWD.” While distinctions between MWD and LWD may exist, the terms MWD and LWD often are used interchangeably. For the purposes of this disclosure, the term LWD will be used generically with the understanding that the term encompasses systems that collect formation parameter information either alone or in combination with the collection of information relating to the position of the drilling assembly.
Ordinarily, a well is drilled vertically for at least a portion of its final depth. The layers, strata, or “beds” that make up the earth's crust are generally substantially horizontal, such as those labeled 20, 21, and 22 in FIG. 1. Therefore, during vertical drilling, the well is substantially perpendicular to the geological formations through which it passes. A sudden measured change in resistivity by a resistivity tool generally indicates the presence of a bed boundary between layers. For example, in a so-called “shaley” formation with no hydrocarbons, the shaley formation has a very low resistivity. In contrast, a bed of oil-saturated sandstone is likely to have a much higher resistivity.
Focusing electrode systems were developed to improve resistivity log response opposite thin beds in high-resistivity formations with low-resistivity borehole fluids. Their major feature is the presence of auxiliary current electrodes above and below the primary current electrodes. These auxiliary electrodes develop potential barriers that cause the primary current to flow into the formation rather than flowing along the borehole. Current flow is focused to travel perpendicular to the borehole wall, as shown in FIG. 2.
More recently, oil-base and synthetic drilling fluids were developed and have become popular to reduce drilling risks and increase efficiency. For example, many wells in locations such as the Gulf of Mexico and the North Sea cannot be drilled as economically using water-based mud technology because of wellbore stability problems. The oil-base muds are highly non-conductive, however, with a resistivity of about 103–106 ohm-m.
Performance of many known micro-resistivity tools in these non-conductive oil-based mud systems was seriously degraded. The high-resistance drilling fluid prevents the flow of current. To solve this problem, a resistivity tool may be placed against the borehole wall, but an imperfect contact or high-resistance mud cake prevents any current flow that would occur perpendicular to the borehole wall. Put in terms of an electrical circuit, the mud cake resistivity Rm (which may be approximated as having infinite resistivity) is in series with the formation resistivity Rt as shown in FIG. 3A. The total circuit resistance may be considered about infinite for this purpose, effectively preventing current flow. Without a flow of current, no resistivity measurement can be made and thus measurements from a current emitting resistivity tool were rendered either useless or much less reliable. It was thought that the advantages of microresistivity borehole imagers to collect information regarding the borehole might be lost when using non-conductive drilling fluids.
Resistivity tools with current flows parallel to the measurement electrodes were developed to overcome this problem. When the current flows parallel to the measurement electrodes along the borehole wall, the electrical circuit analogy is that of a parallel circuit as shown in FIG. 3B. As can be appreciated by those of ordinary skill in the art, a circuit of mud cake resistivity Rm and formation resistivity Rt in parallel results in a circuit resistivity about equal to that of the formation resistivity Rt.
Two resistivity tools developed for use with oil-based (high resistivity) drilling fluid are disclosed in U.S. Pat. Nos. 6,191,588 and 6,348,796, each of which are incorporated by reference. Although the following describes what is believed the salient features of the tools described in these patents, it should be realized that only a shorthand description is contained herein and the patents contain the more full, detailed and possibly more accurate description of each tool design.
U.S. Pat. No. 6,191,588 to Chen describes a resistivity tool designed for use in oil-based mud that places at least two pairs of voltage electrodes between at least one current source and current return on a non-conductive pad, as shown in present FIG. 4. The basic theory of operation underlying the Chen patent is that with an infinitely long pad in perfect contact with formation, the current flow is parallel to the pad in front of the voltage monitors. This allows operation of the device in the oil base mud, in contrast to a focused resistivity tool. Chen then asserts that finite pad size and imperfect pad contact do not significantly impede the parallel flow of current in front of the monitor electrodes and thus the ability of the tool disclosed in the Chen patent to make meaningful measurements.
U.S. Pat. No. 6,348,796 to Evans et al. describes a resistivity tool suitable for use an oil-base mud system. Referring to present FIG. 5 (corresponding to FIG. 4 of Evans), a single measure electrode is maintained at a voltage V. Two guard electrodes flank the measurement electrode and are maintained at voltage below V. The pad or body of the instrument is maintained at a voltage above that of V. This results in a current flow from the measure electrode that is not perpendicular to the borehole wall, a phenomenon that is known as defocusing the current flow. As described by Evans, because of defocusing of the current beam near the measure electrode, the Evans device claims to be relatively insensitive to the presence of borehole fluid between portions of the electrode and the formation.
An alternate resistivity tool is desired that provides accurate measurements of formation resistivity in a non-conductive mud system. Ideally, such a tool would be more simple and more accurate than existing designs.